The parent application shows a method and apparatus for minimizing adverse environmental impact of the production of kraft pulp, or the like, by bleaching the pulp without chlorine or chlorine-containing liquids, and removing metals (such as Mn, Fe and Cu) to prevent buildup of those metals to a level adversely affecting the bleaching reactions. Basically, according to the parent application an acidic liquid stream, such as washer filtrate following a bleach plant acidic stage, is treated with alkaline material and carbonate in a reactor so that the pH is raised to a point where much of the dissolved metal in the filtrate precipitates out, and the precipitated metals are removed by optional clarification and/or filtration. The carbonate controls the calcium level of the filtrate.
According to the present invention, a method and apparatus are provided which improve upon the parent application at least for situations where the bleaching stage just prior to the filtrate wash is not an ozone stage, by oxidizing the filtrate (or other acidic liquid containing dissolved metals) to increase the valence of the metals like Mn and Fe so that they become easier to precipitate or flocculate. That is, by oxidizing a liquid stream according to the invention, the metals precipitate at lower pH's, and the species formed have lower solubility at a given pH than the species predominant in the unoxidized filtrate, making metals removal easier and/or more complete.
According to one aspect of the present invention, a method of removing metals from liquid streams in a cellulose pulp mill is provided, the method comprising the steps of substantially continuously: (a) treating digested pulp in an acidic or neutral stage to dissolve metals; (b) washing or thickening the pulp after step (a) to produce filtrate containing dissolved metals; (c) oxidizing the filtrate to increase the valence of the dissolved metals so that they become easier to precipitate or flocculate; (d) adjusting the pH of the filtrate so that it is at least 6, to cause dissolved metals to precipitate as solids in the pH adjusted filtrate; (e) removing the precipitated solids from the filtrate; and (f) using the reduced metal content filtrate elsewhere in the pulp mill.
Step (e) is typically practiced by filtration, flotation, or sedimentation. Step (c) is typically practiced by bringing a gas selected from the group consisting essentially of air, ozone, oxygen, and combinations thereof, into intimate contact with the filtrate, e.g. using any suitable reactor, such as a plunger type reactor, or a gas-contact reactor, or bringing hydrogen peroxide liquid into contact therewith. Where ozone is used, step (c) is typically practiced at a pH of 2 or more, where oxygen, air, or oxygen-enriched air are used, step (c) is typically practiced at a pH of 4 or more, and when peroxide is used the pH in step (c) is somewhere between. Typically, step (c) is also practiced to provide a filtrate carbonate content of at least about 0.01 mole/l., as by adding carbonate ion so that the filtrate has a carbonate level of approximately 0.05 mole/l.
According to another aspect of the present invention, a method of treating digested kraft pulp is provided, comprising the steps of: (a) oxygen delignifying the kraft pulp; (b) treating the oxygen delignified pulp in an acidic stage, with or without chelation; (c) washing or thickening the pulp after step (b) to produce a filtrate; (d) oxidizing the filtrate to increase the valence of dissolved metals in the filtrate so that the metals become easier to precipitate; (e) adjusting the pH of the filtrate after step (d) to precipitate metals out of the filtrate; and (f) removing the precipitated metals from the pH adjusted filtrate. Steps (d)-(f) may be practiced as described above with respect to the previous aspect of the invention.
According to another aspect of the invention, an apparatus is provided for oxidizing an acidic liquid containing dissolved metals, comprising: (a) means for bringing the acidic liquid into intimate contact with an oxidizing fluid in such a way so as to increase the valence of the metals in the liquid so that they become easier to precipitate, producing a product stream; (b) means for adjusting the pH of the product stream to precipitate metals from the liquid of the stream; and (c) means for removing the precipitated metals from the product stream. The means for removing precipitated metals preferably comprises filtration means, sedimentation means, or flotation means, and the means (a) may comprise any type of conventional reactor that gets intimate contact, such as a gas-contact reactor or a plunger type reactor.
It is the primary object of the present invention to provide for the effective removal of metals from an acidic liquid containing dissolved metals, such as filtrate from a wash or thickening stage following an acidic pulp bleach stage. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.